Sri Lanka is a bit unusual: two separate monsoon systems mean there’s technically a “best time” somewhere on the island almost year-round.
December to March is peak season for the south and west coasts (Mirissa, Weligama, Galle, Colombo), dry, sunny and ideal for beach time and whale watching. This is also when Sri Lanka gets busiest and priciest.
May to September flips the script, this is when the east coast (Arugam Bay, Trincomalee) has its dry season and best surf, while the south and west get their monsoon rains.
The hill country (Kandy, Ella, Nuwara Eliya) sits at higher altitude and stays relatively pleasant most of the year, though it’s cooler and mistier from October to January.
Shoulder months like April and September-October can actually work well for a full-island trip, since neither coast is in full monsoon and prices dip between peak seasons.
Our take: if doing a full loop of the island, timing matters less than most guides suggest, just plan the coastal legs around whichever coast is in its dry season during your trip.